Rapid and precise quality control inspections of the soldering and assembly of electronic devices have become priority items in the electronics manufacturing industry. The reduced size of components and solder connections, the resulting increased density of components on circuit boards and the advent of surface mount technology (SMT), which places solder connections underneath device packages where they are hidden from view, have made rapid and precise inspections of electronic devices and the electrical connections between devices very difficult to perform in a manufacturing environment.
Many existing inspection systems for electronic devices and connections make use of penetrating radiation to form images which exhibit features representative of the internal structure of the devices and connections. These systems often utilize conventional radiographic techniques wherein the penetrating radiation comprises X-rays. Medical X-ray pictures of various parts of the human body, e.g., the chest, arms, legs, spine, etc., are perhaps the most familiar examples of conventional radiographic images. The images or pictures formed represent the X-ray shadow cast by an object being inspected when it is illuminated by a beam of X-rays. The X-ray shadow is detected and recorded by an X-ray sensitive material such as film or other suitable means.
The appearance of the X-ray shadow or radiograph is determined not only by the internal structural characteristics of the object, but also by the direction from which the incident X-rays strike the object. Therefore, a complete interpretation and analysis of X-ray shadow images, whether performed visually by a person or numerically by a computer, often requires that certain assumptions be made regarding the characteristics of the object and its orientation with respect to the X-ray beam. For example, it is often necessary to make specific assumptions regarding the shape, internal structure, etc. of the object and the direction of the incident X-rays upon the object. Based on these assumptions, features of the X-ray image may be analyzed to determine the location, size, shape, etc., of the corresponding structural characteristic of the object, e.g., a defect in a solder connection, which produced the image feature. These assumptions often create ambiguities which degrade the reliability of the interpretation of the images and the decisions based upon the analysis of the X-ray shadow images. One of the primary ambiguities resulting from the use of such assumptions in the analysis of conventional radiographs is that small variations of a structural characteristic within an object, such as the shape, density and size of a defect within a solder connection, are often masked by the overshadowing mass of the solder connection itself as well as by neighboring solder connections, electronic devices, circuit boards and other objects. Since the overshadowing mass and neighboring objects are usually different for each solder joint, it is extremely cumbersome and often nearly impossible to make enough assumptions to precisely determine shapes, sizes and locations of solder defects within individual solder joints.
In an attempt to compensate for these shortcomings, some systems incorporate the capability of viewing the object from a plurality of angles. One such system is described in U.S. Pat. No. 4,809,308 entitled "METHOD & APPARATUS FOR PERFORMING AUTOMATED CIRCUIT BOARD SOLDER QUALITY INSPECTIONS", issued to Adams et al. The additional views enable these systems to partially resolve the ambiguities present in the X-ray shadow projection images. However, utilization of multiple viewing angles necessitates a complicated mechanical handling system, often requiring as many as five independent, non-orthogonal axes of motion. This degree of mechanical complication leads to increased expense, increased size and weight, longer inspection times, reduced throughput, impaired positioning precision due to the mechanical complications, and calibration and computer control complications due to the non-orthogonality of the axes of motion.
Another approach for acquiring shadowgraph X-ray images uses a slit scan geometry with an electronic detector to reduce scattering and interference from adjacent regions of the object being inspected. For example, U.S. Pat. No. 4,383,327 entitled "RADIOGRAPHIC SYSTEMS EMPLOYING MULTI-LINEAR ARRAYS OF ELECTRONIC RADIATION DETECTORS", issued to Kruger describes a scanning radiographic system which uses a multi-linear array operating in a time delay and integration (TDI) mode to generate a slit-scan shadowgraph image of a moving object. Kruger discloses the use of a beam of electronic radiation (e.g., X-rays) generated by a suitable source of electronic radiation. The beam of electronic radiation is directed towards, and aligned with, an array of electronic radiation detectors. Each of the detectors on the array is adapted to generate a signal having a magnitude proportional to the amount of radiation it senses. The array also includes, as an integral part thereof, signal processing capabilities whereby the signals generated by each of the detectors may be stored in respective storage elements. These stored signals, at controlled time intervals, are all shifted to the storage elements of other, adjacent, detectors. Once the signals have been shifted, the signals are augmented by new signals, if any, generated by the respective detectors of the storage elements in which the signals are stored. After having been shifted through several storage elements, these augmented signals may exit from the array to be further processed and conditioned so as to enable an image to be created through a suitable visual system. In connection with the above shifting and processing of radiation signals, the opaque specimen is passed between the source of electronic radiation and the array at a controlled speed and in a known pattern. This controlled speed is synchronized with the controlled time intervals at which the signals are shifted from storage element to storage element. Furthermore, the shifting pattern--that is the sequence that the signals follow as they are shifted from storage element to storage element within the array--is designed to be the same as the movement pattern of the opaque specimen through the beam of electronic radiation. When the shifting pattern of the detector signals is the same as the movement pattern of the opaque specimen, a non-blurred image may be generated. That is, each pixel, or small area, of the image is generated from radiation that passes through a corresponding small area of the specimen. At any instant of time, this radiation falls upon a given detector and generates a signal for that pixel. As the specimen moves, causing the radiation passing through the same small area thereof to likewise move and fall upon an adjacent detector, the pixel signal generated prior to the movement is shifted to the storage element associated with the detector receiving the radiation after the movement. At each storage element, the resolution of the pixel signal is augmented by having it updated to reflect the amount of radiation passing through the corresponding area of the specimen at that particular time. In this fashion, each pixel in the accumulated image results from an integration process. This process is commonly referred to as a time delay and integration (TDI) mode. As shown in Kruger, the angular relationship between the X-ray source, the specific row of image points of the body being examined and the image-recording elements is substantially the same during the production of the X-ray image, i.e., the procedure results in a traditional slit scan transmission X-ray showgraph or radiograph of the object. This TDI (Time Delay and Integration) method of scanning is found to be of particularly good applicability in the examination of bodies by means of X-ray radiation, it being possible for a usable image to be formed despite the fact that each image-recording element, per se, generates only a very small amount of charge in response to the radiation received. A comprehensive discussion of the TDI principle is included in U.S. Pat. No. 4,383,327, the entirety of which is hereby incorporated herein by reference.
The TDI (Time Delay and Integration) mode for operating a CCD camera may also be found in other applications for CCD cameras. For example, U.S. Pat. No. Re. 36,047 entitled "MULTI-MODE TDI/RASTER-SCAN TELEVISION CAMERA SYSTEM", issued to Gilblom et al. describes an optical web inspection system where a CCD operating in a time-delay-and-integration (TDI) mode generates an image of a moving object. U.S. Pat. No. 6,049,584 entitled "X-RAY DIAGNOSTIC APPARATUS FOR PRODUCING PANORAMA SLICE EXPOSURE OF BODY PARTS OF A PATIENT", issued to Pfeiffer describes an apparatus wherein an X-ray source and a CCD detector (having multiple narrow TDI zones) rotate about a patient to produce and sharply image several arbitrarily selectable slices, using a single mechanically executed orthopantomogram. U.S. Patent No. 5,428,392 entitled "STROBING TIME-DELAYED AND INTEGRATION VIDEO CAMERA SYSTEM", issued to Castro et al. describes a TDI camera assembly mounted to view a rotating or other cyclically moving object. The entirety of each of the above referenced patents is hereby incorporated herein by reference.
Many of the problems associated with the conventional radiography techniques discussed above may be alleviated by producing cross-sectional images of the object being inspected. Tomographic techniques such as laminography and computed tomography (CT) have been used in medical applications to produce cross-sectional or body section images. In medical applications, these techniques have met with widespread success, largely because relatively low resolution on the order of one or two millimeters (0.04 to 0.08 inches) is satisfactory and because speed and throughput requirements are not as severe as the corresponding industrial requirements.
In the case of electronics inspection, and more particularly, for inspection of electrical connections such as solder joints, image resolution on the order of several micrometers (for example, a minimum resolved feature size of approximately 20 micrometers (0.0008 inches)) is preferred for current electronic designs. However, better resolution and higher inspection speeds are desirable for inspecting current electronic designs and are rapidly becoming necessary for the inspection of future electronic designs. Furthermore, an industrial solder joint inspection system must generate multiple images per second in order to be practical for use on an industrial production line. Laminography systems which are capable of achieving the speed and accuracy requirements currently necessary for electronics inspection are described in the following patents: 1) U.S. Pat. No. 4,926,452 entitled "AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS", issued to Baker et al.; 2) U.S. Pat. No. 5,097,492 entitled "AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS", issued to Baker et al.; 3) U.S. Pat. No. 5,081,656 entitled "AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS", issued to Baker et al.; 4) U.S. Pat. No. 5,291,535 entitled "METHOD AND APPARATUS FOR DETECTING EXCESS/INSUFFICIENT SOLDER DEFECTS", issued to Baker et al.; 5) U.S. Pat. No.5,621,811 entitled "LEARNING METHOD AND APPARATUS FOR DETECTING AND CONTROLLING SOLDER DEFECTS", issued to Roder et al.; 6) U.S. Pat. No. 5,561,696 "METHOD & APPARATUS FOR INSPECTING ELECTRICAL CONNECTIONS", issued to Adams et al.; 7) U.S. Pat. No. 5,199,054 entitled "METHOD AND APPARATUS FOR HIGH RESOLUTION INSPECTION OF ELECTRONIC ITEMS", issued to Adams et al.; 8) U.S. Pat. No. 5,259,012 entitled "LAMINOGRAPHY SYSTEM AND METHOD WITH ELECTROMAGNETICALLY DIRECTED MULTIPATH RADIATION SOURCE", issued to Baker et al.; 9) U.S. Pat. No. 5,583,904 entitled "CONTINUOUS LINEAR SCAN LAMINOGRAPHY SYSTEM AND METHOD", issued to Adams; and 10) U.S. Pat. No. 5,687,209 entitled "AUTOMATIC WARP COMPENSATION FOR LAMINOGRAPHIC CIRCUIT BOARD INSPECTION", issued to Adams. The entirety of each of the above referenced patents is hereby incorporated herein by reference.
Laminography techniques are widely used to produce cross sectional images of selected planes within objects. Conventional laminography requires a coordinated motion of any two of three main components comprising a laminography system, that is, a radiation source, an object being inspected, and a detector. The coordinated motion of the two components can be in any of a variety of patterns including but not limited to: linear, circular, elliptical or random patterns. Regardless of which pattern of coordinated motion is selected, the configuration of the source, object, and detector is such that any point in the object plane is always projected to the same point in the image plane and any point outside the object plane is projected to a plurality of points in the image plane during a cycle of the pattern motion. In this manner, a cross sectional image of the desired plane within the object is formed on the detector. The images of other planes within the object experience movement with respect to the detector thus creating a blur background on the detector upon which is superimposed the sharp cross sectional image of the desired focal plane within the object.
An example of a laminography system using a circular scan is described in U.S. Pat. No. 4,926,452 entitled "AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS", issued to Baker et al. This patent describes a continuous circular scan laminography system wherein the object remains stationary while the X-ray source and detector move in a coordinated circular pattern. The moving X-ray source comprises a microfocus X-ray tube wherein an electron beam is deflected in a circular scan pattern onto an anode target. The resulting motion of the X-ray source is synchronized with a rotating X-ray detector that converts the X-ray shadowgraph into an optical image so as to be viewed and integrated in a stationary video camera, thus forming a cross sectional image of the object. A computer system controls an automated positioning system that supports the item under inspection and moves successive areas of interest into view. In order to maintain high image quality, a computer system also controls the synchronization of the electron beam deflection and rotating optical system, making adjustments for inaccuracies of the mechanics of the system.
An example of a laminography system using a linear scan is described in U.S. Pat. No. 5,583,904 entitled "CONTINUOUS LINEAR SCAN LAMINOGRAPHY SYSTEM AND METHOD", issued to Adams. This patent describes an improved laminography system that allows generation of high speed and high resolution X-ray laminographs by using a continuous scan method with two or more linear detectors and one or more collimated X-ray sources. Discrete shadowgraph X-ray images, with different viewing angles, are generated by each detector. The discrete X-ray images are then combined by a computer to generate laminographic images of different planes in the object under test, or analyzed in such a manner as to derive useful data about the object under test. In one embodiment, the linear scanning laminography system does not require any motion of the source or detectors, but simply a coordinated linear motion of the object under test. Higher speed is achieved over conventional laminography systems due to the continuous linear nature of the scan and the ability to generate any plane of data in the object under test without having to rescan the object.
In some configurations, cross sectional images for any plane in the object under test may be formed from the data acquired in a single scan. This may be accomplished by combining, e.g., within the data memory of a computer, two or more individual shadowgraph images that were formed during a single scan having coordinated positioning of two of the three main components comprising the inspection system, that is, a source, an object, and a detector. The individual shadowgraph images are combined within the computer memory such that any point in the object focal plane in one individual image is always combined with the same point in the object focal plane of another individual image, this other individual image consisting of a different angular view of the same object. Thus, mathematically shifting the pixel combinations of the multiple individual images has the result of changing the location of the focal plane in the object. For example, the multiple discrete shadowgraph images produced during a single linear scan (as described in U.S. Pat. No. 5,583,904, discussed above) or multiple discrete shadowgraph images produced during a single circular scan by a system such as that described in U.S. Pat. No. 4,926,452 above, may be combined to form a cross sectional image for any selected plane within the test object. Thus, this method of generating a cross sectional image of an object has the advantage over moving and blurring methods in that from one set of shadowgraph images, multiple cross sectional images of different focal planes may be formed. This technique has been called digital tomosynthesis, synthetic laminography, or computerized synthetic cross sectional imaging.
The cross sectional imaging techniques described above are currently used in a wide range of applications including medical and industrial X-ray imaging. Laminography is particularly well suited for inspecting objects which comprise several layers having distinguishable features within each layer. However, some previous laminography systems which produce such cross sectional images typically experience shortcomings in resolution and/or speed of inspection. For example, consider inspection of solder joints for electronic assemblies in a production environment. There are many solder joints to be inspected, and the required inspection time is short. Ideally, the inspection process is in real time, as part of a feedback control system for the manufacturing process. In many manufacturing environments there is a need to verify the integrity of thousands of solder joints within one minute or less. As electronic circuits become more complex and smaller, the size of the solder connections become smaller and the number of solder connections per unit area on the circuit boards increases. In order to keep pace with these changes, solder joint inspection systems must achieve higher resolutions at increased inspection speeds, i.e., decreased time per individual connection.
In general, the above discussed radiographic and laminographic techniques for inspecting solder connections involve various trade-offs such as image quality (approximations, noise, blurring, and artifacts) versus computation time and difficulty of obtaining the required views. Thus, there is an ongoing need for economical systems with improved computation speed while providing suitable image quality. Accordingly, several objects and advantages of the present invention are directed to improved means for achieving high speed and high resolution cross sectional imaging for the inspection of various objects, including electrical connections.